That may be just normal operation. The rear toilet specifically uses a vacuum system to flush because it is not directly over the black tank for gravity flushing. This vacuum system incorporates a pump to move the waste to the black tank; it may also incorporate a macerating pump to accomplish this. After the flush and pump sequence it will then continue to pump to build up a vacuum for the next flush. (when you flush you will hear a sucking sound ..,.. this is the vacuum operating to remove the waste from the toilet. This type os toilet system is common for rear bath RV's.

Yes, I have heard of those systems. Vac-U-Flush or something like that. No electric, No flush!